OVERVIEW

The function of the human eye has been a source of wonder for millennia. What is more, the advancements in modern science that have allowed us to fully characterize the eye functions have led to many more questions than can be answered. By itself, the optical performance of the eye is quite poor. Even in persons with ideal vision, the image falling on the retina (the eye's detectors) has aberrations. Parallel lines appear to be curved toward and away from one another, the image is inverted relative to the objects in the scene (as in most other imaging systems) and, above all, there is a point at which there is no vision at all. Most surprising is that everywhere in the retina, except at the fovea (the area of highest acuity), light has to pass through a number of layers of tissue and blood vessels to get to the photosensitive material. In the area of the fovea, there is a "pit" where the overlayers are not present. The exposed cones in the fovea have a density five to six times higher than in the periphery. The individual cones are about 3 ?m in extent and hexagonal in shape.

Fortunately, the optical system is just the beginning of the vision system. Behind the eye is the most powerful computer known to man, and it corrects virtually all of these faults although a pair of glasses or contact lenses are often invaluable in providing the finishing touch on the system.

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